1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
As a nonaqueous electrolyte secondary battery for a portable electronic appliance such as a portable telephone, a thin lithium ion secondary battery is being commercialized in recent years. Proposed as such a thin lithium ion secondary battery is a battery comprising an electrode group comprising a positive electrode, a negative electrode and a separator interposed between the positive and negative electrodes. The electrode group is impregnated with a nonaqueous electrolyte and is housed in a case formed of a laminate film so as to provide the thin lithium ion secondary battery. The active material contained in the positive electrode of the secondary battery is formed of, for example, a lithium complex oxide, and a carbonaceous material capable of absorbing-desorbing lithium ions is contained in the negative electrode. It should be noted that, during storage of the charged secondary battery of the particular structure under high temperatures, the nonaqueous electrolyte tends to react with the positive electrode, with the result that the nonaqueous electrolyte tends to be decomposed by oxidation so as to generate a gas. What should be noted is that the case formed of a laminate film is swollen by the gas generation so as to do damage to the electronic appliance. In order to suppress the deformation of the case caused by the gas generation, it is conceivable to use a nonaqueous electrolyte prepared by dissolving a solute in a nonaqueous solvent containing γ-butyrolactone.
It should be noted that a protective film called an SEI (Solid electrolyte interphase) is formed on the surface of the negative electrode so as to ensure a sufficient charging-discharging of the negative electrode. However, where the secondary battery is stored under a high temperature environment for a long time, the SEI is denatured by some reasons so as to impair the stability of the negative electrode. In such a case, the self-discharge tends to be increased.